This invention relates generally to voice coil actuators and, more particularly, to magnetic devices used in voice coil actuators.
The use of voice coil actuators is well known. In common magnetic circuits of voice coil actuators, a current-carrying coil travels in the air gap and is subject to a net magnetic force. For example, in FIG. 1, a current-carrying coil 10 travels along the direction of axis X-X in the air gap 20 and is subjected to a net magnetic force by magnet 30. The force generated is proportional to the number of turns of wire in coil 10, the amount of current flowing in the wire of coil 10, and the strength of the magnetic field, in this case caused by magnet 30 in air gap 20, through which coil 10 passes. As coil 10 traverses the length of the actuator, it is desirable to maintain a uniform force, in the direction of travel, imparted on coil 10 with a constant current.
However, many actuator designs do not provide the desired uniform force characteristic. This non-uniformity may be caused by the inappropriate design of the air gap between the magnet and the return structure at the longitudinal ends of the actuator, causing the effective magnetic field of the magnet to decrease near the magnet""s ends. For example, as shown in the typical voice coil actuator of FIG. 2, a magnet 30 is enclosed by a return structure 50 such that there is an air gap 20. The lack of an appropriately designed air gap causes flux lines 70 of magnet 30 near the ends of the magnet to xe2x80x9cshort,xe2x80x9d that is, to turn towards the ends (first return structure end 52 and second return structure end 54) of return structure 50 rather than to travel completely across air gap 20. This effect causes a significant drop in the useful magnetic flux component of the magnet when the coil""s travel nears the ends of the magnet. Since the actuator""s generated force is proportional to magnetic flux and current, in constant current devices this drop-off in effective flux results in an undesirably low force at the ends of coil""s travel. In current-compensated devices, the flux drop-off results in undesirably high currents being required to maintain nominal coil force at the ends of the coil""s travel.
One solution known in the art is to increase the thickness of the magnet near its ends to compensate for flux-line shorting. This modifies the air gap length near the magnet ends and boosts the flux in the air gap by reducing the length of air gap across which the flux lines must travel. The thickness of the magnet may be increased in either step and/or ramp fashions. An example of a typical ramp-type solution is shown in FIG. 3, where the air gap 20 in return structure 50 has a shorter length X near the end 52 of return structure 50 due to the increased thickness Y of ramped magnet 90. It will be appreciated that in such known magnet designs, all portions of the top surface of the magnet are either horizontal (surfaces 92 and 96) or have a positive slope (surface 94) ramping upwards towards the magnet end. However, the drop-off in resultant actuator force when the coil""s position nears the magnet""s end is still significant in actuators having the ramp-type solution. This can be seen in the graph of FIG. 4, which shows, for a typical voice-coil actuator employing such a solution, the resultant force (F) as a function of the position of the voice coil actuator""s current-carrying coil in relation to its magnet as well as the percentage variation (P) from the maximum force. It can be seen that the decrease from the maximum force exceeds 4% near the ends of the actuator""s travel.
Accordingly, there is a need in the art for a voice coil actuator that can minimize the effective drop-off of its resultant force when the position of the coil nears one or both ends of the magnet.
The present invention is directed to an improved magnet for use in voice coil actuators. The magnet includes first and second ends and a top surface, the top surface including a reversed slope notch located in proximity to the first end of the magnet. The top surface may also include a second reversed slope notch located in proximity to the second end of the magnet. The magnet may further include additional reversed slope notches, positioned in proximity to either or both the first and second magnet ends. The reversed slope notch or notches may be located such that the magnet is shaped symmetrically about its center. In one embodiment, the magnet may be part of a linear voice coil actuator that includes a return structure that encloses the magnet and an air gap. The top surface of the magnet is located adjacent the air gap, and a current-carrying coil travels through the air gap.
In another embodiment, a magnet for use in a voice coil actuator includes first and second magnet ends and a top surface and a bottom surface, the top surface including a first reversed slope notch in proximity to the first magnet end. The magnet may include a second reversed slope notch in proximity to the second magnet end. The magnet may include a magnet top end surface, wherein the reversed slope notch includes first and second notch surface configured such that the angle between the end surface and the first notch surface and the angle between the end surface and the second notch surface are both greater than zero degrees. The bottom surface of the magnet may be co-planar with a horizontal plane, and the reversed slope notch of the magnet may include first and second notch surfaces, where the angle between the horizontal and the first notch surface and the angle between the horizontal and the second notch surface are both greater than zero degrees. The reversed slope notch may include first and second notch surfaces and a vertex, the first notch surface being on the side of the apex closer to the first magnet end, the second notch surface being on the side of the apex distant from the first magnet end and sloping downwards towards the first magnet end. The reversed slope notch may include first and second notch surfaces, the angle subtended by the first and second notch surfaces being acute. The magnet may include at least one additional reversed slope notch positioned near the first magnet end, and the thickness of the magnet, excluding the effect on the thickness resulting from the notches, may increase towards the first magnet end. Each additional reversed slope notch may include first and second notch surfaces associated with that notch, wherein peaks are formed by the intersection of the first and second notch surfaces of adjacent notches. The height of each of the peaks may vary inversely with the distance of each of the peaks from the first magnet end. The angle subtended by the first and second notch surfaces of each successive reversed slope notch may increase with the distance from the first magnet end.
In another embodiment, a voice coil actuator including a magnet, includes-a return structure including first and second return structure ends and a bottom portion, the return structure enclosing a magnet and an air gap; the magnet comprising: a first magnet end positioned adjacent the first return structure end; a second magnet end adjacent the second return structure end; a top surface positioned adjacent the air gap; a bottom surface positioned adjacent the bottom portion; and at least one reversed slope notch. The reversed slope notch may be formed by a first notch surface and a second notch surface, the first notch surface being substantially planar and substantially perpendicular to the horizontal, the second notch surface being substantially planar and meeting the first notch surface to subtend an angle. The voice coil actuator may firer comprise a second reversed slope notch in proximity to the second magnet end. The voice coil actuator may include a magnet top end surface, wherein the angle between the end surface and the first notch surface and the angle between the end surface and the second notch surface are both greater than zero degrees. The bottom surface of the magnet may be co-planar with a horizontal plane, and the angle between the horizontal and the first notch surface and the angle between the horizontal and the second notch surface may both be greater than zero degrees. The reversed slope notch may have a vertex where the first and second notch surfaces intersect, the first notch surface being on the side of the apex closer to the first magnet end, the second notch surface being on the side of the apex distant from the first magnet end, and the second notch surface sloping downwards towards the first magnet end. The angle subtended by the first and second notch surfaces is acute. The voice coil actuator may include at least one additional reversed slope notch positioned near the first magnet end, and the thickness of the magnet, excluding the effect on the thickness resulting from the notches, may increase towards said first magnet end. Each additional reversed slope notch may include first and second notch surfaces associated with that notch, wherein peaks are formed by the intersection of the first and second notch surfaces of adjacent notches. The height of each of the peaks may vary inversely with the distance of each of the peaks from the first magnet end. The angle subtended by the first and second notch surfaces of each successive reversed slope notch may increase with the distance from the first magnet end. The voice coil actuator may include a current-carrying coil traveling through the air gap. The voice coil actuator may be a linear voice coil actuator. The voice coil actuator may include a ramp positioned in proximity to the first magnet end.
In another embodiment, a linear voice coil actuator includes a magnet; a return structure including first and second return structure ends and a bottom portion, the return structure enclosing the magnet and an air gap; a current-carrying coil traveling through the air gap; wherein the magnet includes: a first magnet end positioned adjacent the first return structure end; a second magnet end positioned adjacent the second return structure end; a top surface positioned adjacent the air gap; a bottom surface positioned adjacent the bottom portion; a reversed slope notch formed by a first notch surface and a second notch surface, the first notch surface being substantially planar and substantially perpendicular to the horizontal, the second notch surface being substantially planar and meeting the first notch surface to subtend an angle; and a ramp positioned in proximity to the first magnet end.
In another embodiment, a magnet for use in a voice coil actuator includes first and second magnet ends and a top surface and a bottom surface, the top surface including a first reversed slope notch in proximity to the first magnet end and a second reversed slope notch in proximity to the second magnet end